When a vehicle is in distress, radio is often used to communicate that fact to others and to summon assistance. For example, on the high seas when a ship is in grave danger it will broadcast the international distress signal on dedicated distress frequencies upon which all vessels maintain watch. If a vessel receives a distress signal, it is obligated to render such assistance as may be required and to deviate from its planned course if necessary. Of course, there is little to guarantee that a transmitted distress signal will be received or, if it is received, that the recipient will render assistance.
On a smaller scale, persons or vehicles within the limits of a city may also find themselves in danger. Here, however, since most land-based travelers are not equipped with radio stations, the distress call is made by any available means, such as shouting. Once again, for a number of reasons, the call may go unanswered.
For these persons or vehicles (especially vehicles transporting valuable cargoes), more reliability may be desired in the response to a distress signal; and the desired level of reliability may justify providing a distress radio transmitter. Such a transmitter for personal use is described in Hull, U.S. Pat. No 3,440,635 (Apr. 22, 1969). Hull describes a personal alarm transmitter to be used by a crime victim to summon police to the scene. When the victim surreptitiously activates his concealed transmitter, police, both at fixed locations and in automobiles, use direction-finding equipment to locate the source of the signal. The transmission may be modulated to indicate the transmitter's approximate location (in the event that it is normally located in a particular area) or its identity.
Since direction-finding (DF) equipment is used, all that is needed to determine the direction of the transmitter from the receiver is the presence of a radio signal on the distress frequency. Two or more sets of DF equipment can then locate the transmitter. One difficulty with DF is that it requires a signal of relatively long duration (or one which is repeated many times) in order to obtain a fix, whereas, in an emergency, it may not be possible to transmit more than a signal of short duration.
The Hull system suffers from other drawbacks: First, it is not a dedicated system. The police, in addition to listening for distress radio transmissions, have other duties which may interfere with locating the distress transmitter (or even take precedence over it). Also, the Hull system is slower and less effective because it relies to a great extent on actions taken by humans rather than actions performed automatically.
Some prior art systems which do not use DF for navigational purposes use instead the signal strength of a transmitted signal. (By "signal strength" is meant the field intensity or flux density of the transmitted energy.) For example, Frenkel U.S. Pat. No. 3,922,678 (Nov. 25, 1975) discloses an arrangement which may be used to locate emergency radio transmissions in an area, such as a city, where numerous receivers have been pre-positioned at intersections of a coordinate grid. The sensitivity and location of the receivers, and the power output of the emergency transmitter, are adjusted so that (a) a transmission at any point in the area of coverage will be received by at least two receivers, and (b) each combination of receivers at which a given transmission can be received corresponds to a unique zone within that area. The transmitter is activated either by a police officer when in pursuit of a crime suspect or by police intending to track a package in the possession of a suspect. Each receiver in the area of coverage is connected to a relay station which relays the fact of receipt or non-receipt of the transmitter's signal to the police station, where the transmitter's path can be followed.
The Frenkel system, while providing some degree of automatic radiolocation capacity, is unable to locate a transmitter precisely because it divides the area of coverage into zones and only determines, for each zone, whether a transmitter is present there. For greatly increased accuracy, a much larger number of zones (and receivers) would be required. Furthermore, Frenkel does not include means for rescuing a crime victim who is sending an emergency transmission. The system merely follows, from a central station, the progress of the transmitter throughout the city. Nor would the system disclosed be suitable for personal emergency rescue, for two reasons. First, the transmitter must operate continuously in order to allow the operator at the central station to monitor its location. Second, transmitter power must be precisely controlled because the accuracy of the system (limited as it is) is strictly dependent upon the size and shape of the receiver fields, which must remain fixed.